A ball spline device, as disclosed in U.S. Pat. No. 5,809,838, includes a screw shaft, a ball nut sleeved on the screw shaft, a first ball unit fitted between the screw shaft and the ball nut, a rolling bearing sleeved on the ball nut, and a second ball unit disposed between the ball nut and the rolling bearing. The rolling bearing has a first oil supplying hole formed in an outer race thereof. The ball nut has an inner peripheral surface, an outer peripheral surface, and a plurality of second oil supplying holes communicating with the first oil supplying hole through an annular gap. When lubricant oil is introduced into the ball spline device via the first oil supplying hole, the lubricant oil must fill up the annular gap before the lubricant oil can flow through the second supplying holes to a space between the screw shaft and the ball nut, thereby resulting in excessive oil and poor lubrication efficiency. Further, this type of ball spline device cannot produce a good lubricating effect for the first and second ball units.
Another ball spline device, as disclosed in Taiwanese Patent Publication No. 1444542, includes a screw shaft, a ball nut sleeved on an outer portion of the screw shaft, a first rolling ball unit disposed between the screw shaft and the ball nut, a bearing outer ring sleeved on the ball nut, and a second rolling ball unit disposed between the bearing outer ring and the ball nut. The ball nut has two spaced-apart protruding portions, an annular oil channel disposed between the protruding portions, and a plurality of oil holes extending radially through an inner peripheral face from the annular oil channel. The bearing outer ring has an oil supplying hole communicating with the annular oil channel. Although the provision of the protruding portions can prevent excessive lubrication of the lubricant oil on the second rolling ball unit, because the protruding portions are spaced apart from each other, the second rolling ball unit is not sufficiently lubricated.